Ion-selective electrodes (ISEs) are widely used in various application fields, including clinical analysis, process control, and environmental monitoring (Bakker et al. 1997; Bühlmann et al. 1998; Bobacka et al. 2008; Bühlmann et al. 2012; Johnson et al. 2003). To achieve sensor miniaturization, small sample volumes, easy maintenance, and scalability for mass production, solid-contact ion-selective electrodes (SC-ISEs), in which a solid contact is used as the ion-to-electron transducer, have attracted much attention (Bobacka et al. 2008; Linder et al. 2009; Michalska 2012; Pretsch 2007). In view of the need for affordable and portable analytical devices for small sample volumes, miniaturizable SC-ISEs are highly desirable but are only meaningful if the reference electrode is also miniaturized.
The first proposed SC-ISE, the coated-wire electrode was extremely simple but unreliable due to the ill-defined interfacial potential between the ion-selective membrane (ISM) and the underlying conducting metal (Cattrall et al. 1971). To stabilize this interfacial potential, intermediate layers consisting of conducting polymers with high redox capacitance, such as derivatives of polypyrrole, polythiophene, and polyaniline, were introduced (Cadogan et al. 1992; Bobacka et al. 1994; Bobaca et al. 1995). Some of these sensors have shown interference from gases or are affected by the build-up of an unintended water layer between the ion-selective membrane (ISM) and the solid contact (Vazquez et al. 2002; Fibbioli et al. 2000). More importantly, since these conducting polymers are polydisperse and exhibit a continuum of local geometries, they do not have a well-defined redox potential. Consequently, it is difficult to obtain high device-to-device reproducibility and to minimize long-term drift due to reactions of the conducting polymer with ambient redox-active species such as oxygen.
Conventional reference electrodes are typically Ag/AgCl or Hg/Hg2Cl2 half cells and are connected to the sample through a salt bridge. The latter usually contains an aqueous solution of an equitransferent salt that minimizes the liquid junction potential at the interface of the bridge electrolyte and the sample. Although very stable and reliable, such reference electrodes exhibit disadvantages owing to the presence of the salt bridge, such as the need for frequent maintenance, a large size, and the mutual contamination of the bridge electrolyte and sample.
More recently, nanostructured carbon materials such as three-dimensionally ordered macroporous (3DOM) carbon, carbon nanotubes, fullerene, and graphene have gained the attention of electrochemists due to their intrinsic hydrophobicity and electric conductivity (Lai et al. 2007; Fierke et al. 2010; Crespo et al. 2008, 80; Crespo et al. 2008, 81; Fouskaki et al. 2008; Ping et al. 2011; Hernindez et al. 2012; Li et al. 2012). SC-ISEs based on these carbon materials have exhibited few problems with water layer formation and interference by O2, CO2, or light. Among the sensors with one of these carbon materials as an interlayer, the 3DOM carbon-based SC-ISEs have shown the most favorable long-term potential stability, which can be explained by the high capacitance of the interface between this carbon material and the ISM.
3DOM carbon consists of a glassy carbon skeleton with interconnected macropores that can be infiltrated with the ISM to form a bicontinuous ion- and electron-conducting structure. Its large interfacial contact area and high capacitance lead to excellent long-term stability of 3DOM carbon-based SC-ISEs, with a drift as low as 11.7 μV/h (Lai et al. 2007; Fierke et al. 2010). With these sensors, a subnanomolar detection limit of Ag+ and trace-level detection of perfluorinated surfactants in lake water have been achieved (Lai et al. 2009; Chen et al. 2013). However, 3DOM carbon prepared from resorcinol-formaldehyde precursors contains significant amounts of redox-active surface functional groups that can affect the reproducibility of the calibration curve intercept, E°. As a consequence, SC-ISEs that use 3DOM carbon still require calibration. Moreover, the monolithic nature of 3DOM carbon as used in the past is problematic in view of mass production of sensors (Fierke et al. 2010).